The invention relates to an apparatus for measuring and/or monitoring a constituent of an atmosphere by means of a thermosensitive element fed electrically and disposed in a porous-wall measuring cell.
Measuring and/or monitoring instruments, such as explosimeters, methanometers, katharometers, or other devices for determining constituents of an atmosphere, are known, in which a measuring cell containing at least one electrically heated measuring filament or sensitive element is disposed in the atmosphere. In known automatic instruments which are permanently installed it is known that the display of the value of the amount of the constituent being monitored is subject to a delay due to the diffusion of the atmosphere through the porous wall.
This delay does not usually constitute a serious disadvantage, because the use of such instruments is restricted to the monitoring of places where the variation is slow and the amounts monitored are generally remote from the danger threshold, it being most important to detect an increase in the amount of the monitored constituent, for example in the case of explosimetry.
In addition, U.S. Pat. No. 4,002,429 discloses an intermittent measuring instrument in which the measuring time is delayed in relation to the moment T.sub.o at which a voltage is applied and at which the gas to be measured is introduced. French Pat. No. 2,337,339 (equivalent to U.S. patent application 645,838 now abandoned) describes an intermittent interrogation apparatus in which storage in a memory permits permanent reading of the inflammable gas content. If devices of these kinds were applied to a portable porous-wall cell apparatus, disadvantages would result which will be explained below.
It will first be recalled that the porous-wall cell has two important advantages, namely, that the wall can be made of sintered metal possessing great mechanical strength for the protection of the fragile filament, and that no mechanical means is required for pumping the atmosphere into the measuring cell.
If it is desired to make use of these advantages for a manual measuring instrument, the operator must be given instructions to postpone his reading until the time required for the diffusion of the atmosphere into the cell has expired.
If however, the operator does not conscientiously follow these instructions, or if he is distracted, he may not wait long enough for good diffusion of the gas to occur. This will result in erroneous measurements.
It is an object of this invention to propose an apparatus avoiding this disadvantage and ensuring that the measurement is made at the most convenient moment both for achieving good diffusion of the gas and for obtaining the most accurate reading possible.